HOMOGENEITY REGION AND THERMAL-STABILITY OF NEODYMIUM-DOPED ALPHA-SIALON CERAMICS

Dense sialon ceramics along the tie line between Si3N4 and Nd2O3 . 9Al N were prepared by hot-pressing at 1800 degrees C, The materials were subsequently heat-treated in the temperature range 1300-1750 degrees C and cooled either by turning off the furnace (yielding a cooling rate (T-cool) of approximate to 50 degrees C/min) or quenching (T-cool gre ater than or equal to 400 degrees C/min), It was found necessary to us e the quenching technique to reveal the true phase relationships at hi gh temperature, and it was established that single-phase alpha-sialon forms for 0.30 less than or equal to x less than or equal to 0.51 in t he formula NdxSi12-4.5xAl4.5xO1.5xN16-1.5x. The alpha-sialon is stable only at temperatures above 1650 degrees C, and it transforms at lower temperatures. by two slightly different diffusion-controlled processe s, Firstly, an alpha-sialon phase with lower Nd content is formed toge ther with an Al-containing Nd-melilite phase, and upon prolonged heat treatment thus-formed alpha-sialon decomposes to the more stable beta- sialon and either the melilite phase or a new phase of the composition NdAl(Si6-zAlz)N10-zOz. Nd-doped alpha-sialon ceramics containing no c rystalline intergranular phase show very high hardness (HV10 = 22.5 GP a) and a fracture toughness (K-1C = 4.4 MPa . m(1/2)) at room temperat ure, The presence of the melilite phase, which easily formed when slow cooling rates were applied or by post-heat-treatment, reduced both th e fracture toughness and hardness of the materials.